Leaky cables (e.g. radiating cables, leaky feeders) are used in wireless cellular systems to provide improved coverage, especially in the case of tunnels or along railways but also in indoor deployments. The leaky cable acts as a very long antenna, which can help in obtaining a more uniform coverage level, compared to a single (small) antenna from which the radiated power falls off rapidly with distance. FIG. 1 depicts a comparison of the coverage of a leaky cable (on the left) and a point source antenna (on the right). The system has a limited range and because of the high frequency it uses, signal transmissions cannot pass through solid rock, which usually limits the system to line of sight applications.
A leaky feeder is typically designed as a coaxial cable (waveguide) where the outer conductor is perforated in order to create holes or slots through which some of the energy in the cable can escape and radiate into free space. Various designs exist for the slot geometry and separations, slots can be uniformly distributed along the length of the cable, or clustered in groups, thereby providing different radiating properties. Variations of the slot structure, shape, and density along the cable allow a cable designer to shape how much the cable is radiating from different sections and in what directions. The latter property is realized through selecting on which side of the cable the slots are placed, as each slot will have more or less pronounced directional radiation properties that essentially form a lobe or beam away from the cable. An example of a commercial leaky feeder (coaxial cable with radiating slots) is shown in FIG. 2. It has been found through measurements and numerical simulations that a leaky feeder such as the one depicted in FIG. 2 will have its radial radiation maximum in the direction that the slots are facing.
While the cable designer has plenty of freedom when designing the cable, it is next to impossible to provide a design that is optimal for a given installation since it is unknown beforehand where the cable will be installed. For instance, there might be sections along the cables length where it is undesirable that it radiates, such as where it passes through walls, floors, or cable ducts. Similarly, the orientation of the cable with respect to nearby structures such as walls, supports, and other cabling might be impossible to predict. Even if the preferred orientation is known, it might be difficult to achieve due to the cable rigidity and installation paths with curves and corners. Nearby metallic objects might partially cover the slots causing less radiation to escape from the cable, or lossy materials such as concrete walls may heavily attenuate the radiation.
The first problem is exemplified in FIG. 3 where a leaky feeder (leaky cable) is utilized to illuminate three separate areas or rooms, as indicated by the white squares. These areas could e.g. represent different rooms or floors in a building, or different tunnel sections. The surrounding area (between the rooms) represents parts of the installation area where radiation is undesirable, such as concrete walls or cable ducts where any radiation will be heavily attenuated and therefore not usable for communication. A cable that is radiating in these areas will therefore radiate less energy in the coverage areas. The dotted radiation lobes in FIG. 3 indicate this.
The second problem is exemplified in FIG. 4. A leaky feeder cable (leaky cable) is typically mounted on e.g. a wall as depicted in the figure. Inappropriate orientation of the cable close to a conductive object, as depicted by the black square in the upper part of the figure, may lead to lower radiation efficiency, since the slots are essentially covered by the conductive object. Similarly, inappropriate orientation close to a lossy object, as illustrated by the wall in the lower part of the figure, may lead to more attenuation of the radiated power. In both cases, less energy is radiated in the direction of the intended coverage area (as indicated by the arrow) compared to an optimal orientation of the cable.
Based on the above discussion, there is a need to provide a leaky feeder cable that supports a more optimal coverage and reduces the occurrence of the leaky cable radiating in undesirable directions or locations along its installed path.